In an electromagnetic flow meter of the type described in French Pat. No. 2 405 466, the liquid whose flow rate is to be measured flows along a portion of duct which is fitted with induction windings to set up a magnetic field perpendicularly to a portion of duct, and a pair of diametrically opposed electrodes for sensing the voltage which results from the liquid flowing through the magnetic field. This voltage is proportional to the average speed of the liquid and thus to its average flow rate. The voltage can be processed to obtain an electric signal representative of the liquid flow rate.
The magnetic field may be set up either by means of a sinewave current or else by means of a pulsed direct current.
When using a sinewave current, the frequency of the signal is generally about 30 Hz to about 50 Hz. The signal sensed on the electrodes comprises: a flow rate signal which is in phase with the modulation of the electric current, a parasitic induction signal which is in quadrature with the modulation of the current; and a second parasitic signal whose phase and amplitude vary as a function of the nature of the fluid and the surface state of the electrodes.
Such a system suffers from drift even when the flow rate is zero. This drift may be as much as several percent depending on the type of fluid and on the electrodes. The zero point adjustment must be performed on the final installation by the user. However, with this type of drive to the magnetic field, it is easy to filter out very low frequency noise (&lt;10 Hz) created by particle-laden resistive liquids and by liquids having marked acid or basic properties. Given the frequency of the base signal (30 Hz to 50 Hz), effective filtering can be performed with a short time constant, for example less than three seconds. Such a time constant is compatible with the flow meter being used as a link in a regulation servo system, for example.
When controlled by a pulsed current, the modulation frequency is generally chosen to lie in the range 1 Hz to 10 Hz. Accompanying FIGS. 1a and 1b show the operation of prior art flow meters using pulsed current control.
FIG. 1a shows the intensity I of the control current (or field strength B of the resulting magnetic field) as a function of time t. FIG. 1b shows the corresponding voltage signal (V) sensed on the electrodes of the flow meter as a function of time, for a given flow rate of a given liquid.
Each pulse in the voltage signal comprises: a first zone A corresponding to a parasitic induction effect which occurs when the direction of the magnetic field is reversed; followed by a zone B corresponding to a relaxation effect produced in the fluid-electrode interface at the moment the magnetic field is reversed. As shown by the curve in FIG. 1b, this parasitic effect wears off slowly to leave a pure flow rate signal which corresponds to a zone C. In order to obtain a signal representative of the flow rate, the signal must be observed in zone C of pulse. The flow rate signal is then proportional to U.sub.1 -U.sub.2. This system has a highly stable zero point, and in addition, any drift from zero is self-compensating since the measurement signal comes from the difference of two voltages, thereby eliminating a parasitic DC voltage referred to as the asymmetry voltage. FIG. 1b does not show the asymmetry voltage. The above-mentioned French patent describes in detail how the asymmetry voltage may be eliminated. In contrast, this feed system makes it difficult to properly eliminate low frequency noise of the type present in non-homogenous sludge. Given the frequency of the signal to be filtered (1 Hz to 10 Hz), the time constant of the filter circuit may be as much as several tens of seconds. This makes it very difficult, if not impossible, to use such a flow meter in a regulation control loop, since obtaining a noise-free measurement signal gives rise to a response time which is too long.
Preferred embodiments of the present invention provide an electromagnetic flow meter which combines the advantages of the two prior art principles in order to measure flow rates without zero drift and with low noise combined with a rapid response time. Such a flow meter is thus easier to use in a regulation chain, for example.